17 Questions
What is the main advantage of using a shunt resistor in a moving coil ammeter?
It increases the current capacity of the instrument
Why is a permanent-magnet moving coil (PMMC) instrument impractical for measuring large currents directly?
The current capacity of the PMMC is too small
What is the relationship between the shunt resistance ($R_{sh}$) and the internal resistance of the movement ($R_m$) in a moving coil ammeter?
$R_{sh} = R_m / (I / I_m - 1)$
Which of the following is NOT a desirable property of a shunt resistor for a moving coil ammeter?
It should have a high thermal electromotive force with copper
What is the purpose of the 'multiplying power of the shunt' ($I/I_m$) in a moving coil ammeter?
It converts the current reading to the actual current being measured
How can the current range of a moving coil ammeter be extended to measure higher currents?
By connecting a shunt resistor in parallel with the instrument movement
What is the main advantage of a moving coil instrument compared to other types of instruments?
Greater sensitivity and precision
Which of the following equations correctly represents the relationship between the full-scale voltage ($V$), the deflection current ($I_m$), the internal resistance ($R_m$), and the series resistance ($R_s$) for a voltmeter?
$V = I_m (R_s + R_m)$
What is the purpose of using a shunt resistor in an ammeter?
To increase the range of the instrument
Which of the following equations correctly represents the relationship between the shunt resistance ($R_{sh}$), the internal resistance ($R_m$), and the full-scale deflection current ($I_m$) for an ammeter?
$R_{sh} = R_m / (I_m / (I_m - 1))$
What is the main disadvantage of using a moving coil instrument for high-voltage measurements?
Requirement of a multiplier resistor
Which of the following equations correctly represents the relationship between the full-scale voltages ($V_1$, $V_2$, $V_3$, $V_4$) and the internal resistance ($R_m$) for a multi-range voltmeter?
$V_2 = V_1 - I_m R_m$, $V_3 = V_2 - I_m R_m$, $V_4 = V_3 - I_m R_m$
What is the main purpose of the two springs being spiraled in opposite directions in a moving coil instrument?
To neutralize the effects of temperature changes
Which of the following is NOT an advantage of a permanent-magnet type moving coil instrument?
Ability to be modified to cover a wide range of currents and voltages
Which of the following is a disadvantage of permanent-magnet type moving coil instruments compared to moving-iron instruments?
More delicate construction and costlier
What is the main purpose of using a shunt with an ammeter?
To extend the range of the ammeter
Which of the following is NOT a desirable property of a good shunt for an ammeter?
Uniform scale
Study Notes
Ideal Shunt Characteristics
- The resistance of a shunt should not vary with time.
- Shunts should carry current without excessive temperature rise.
- They should have a thermal electromotive force (EMF) with copper.
Ammeter Shunt
- A PMMC (Permanent Magnet Moving Coil) is used as the indicating device in an ammeter.
- The current capacity of a PMMC is small, making it impractical to construct a PMMC coil that can carry a current greater than 100 mA.
- A shunt is required for measuring large currents in an ammeter.
Shunt Calculation
- The voltage drop across the shunt (Rsh) and movement (Rm) must be the same.
- The equation to calculate the shunt resistance (Rsh) is: Rsh = Im * Rm / (I - Im)
Multi-Range Ammeter
- A multi-range ammeter uses multiple shunts with different multiplying powers (m) to measure different ranges of currents.
- The shunt resistance for each range is calculated using the equation: Rsh = Rm / (m - 1)
Voltmeter
- A series resistor or multiplier is required to extend the range of a voltmeter.
- The equation to calculate the multiplier resistance (Rs) is: Rs = (V - Im * Rm) / Im
Multi-Range Voltmeter
- A multi-range voltmeter uses multiple multipliers with different multiplying powers (m) to measure different ranges of voltages.
- The multiplier resistance for each range is calculated using the equation: Rs = Rm * (V - Im * Rm) / Im
Examples
- Example 1: Calculating the shunt resistance for an ammeter with a full-scale deflection current of 1 mA and an internal resistance of 100 ohms.
- Example 2: Calculating the series resistance for a voltmeter with a full-scale deflection current of 1 mA and an internal resistance of 100 ohms.
- Example 3: Calculating the full-scale reading of a PMMC instrument.
PMMC Instrument Advantages
- They have low power consumption.
- Their scales are uniform and designed to extend over an arc of 170°.
- They possess a high (torque/weight) ratio.
- They can be modified with the help of shunts and resistances to cover a wide range of currents and voltages.
- They have no hysteresis loss.
- They have very effective and efficient eddy-current damping.
- They are not much affected by stray magnetic fields.
PMMC Instrument Disadvantages
- They are somewhat costlier due to delicate construction and accurate machining and assembly of various parts.
- Errors set in due to the aging of control springs and the permanent magnets.
- They are mainly used for DC work only, but can be used for AC measurements with rectifiers or thermo-junctions.
Learn about the important characteristics and functions of shunt resistors used in ammeters. Understand why shunt resistors should not vary with time, carry current without excessive temperature rise, and have a thermal electromotive force with copper. Explore the necessity of using shunt resistors to measure large currents in ammeter circuits.
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